1. Field of the Invention
This invention relates to electromagnetic detection systems and more particularly it concerns novel arrangements which provide improved performance from such systems.
2. Description of the Prior Art
The prior art electromagnetic detection systems to which the present invention applies are shown and described in French Pat. No. 763,681 issued on Feb. 19, 1934 to Pierre Arthur Picard. As disclosed in that patent, an article to be detected, e.g., a library book, is provided with a target or marker of a special saturable magnetic material, such as permalloy. A large coil is arranged near a doorway or other egrees passageway leading from the area where the article to be detected is kept. This coil is energized with an alternating electrical signal so that it produces a corresponding alternating interrogation magnetic field in the region of the doorway. When the book bearing the permalloy marker passes through the alternating interrogation magnetic field, as when it is carried out through the doorway, the permalloy marker converts a portion of the alternating interrogation magnetic field energy to other alternating magnetic fields at various harmonics of the frequency of the fundamental or interrogation magnetic field. Selected ones of these harmonics are detected in the receiver; and when these harmonics are detected an alarm is sounded.